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Physiology IllustrationPrimary and secondary active transporters in membranes of secretory and synaptic vesicles.

Primary and secondary active transporters in membranes of secretory and synaptic vesicles.

While many primary and secondary active transporters are localized to the plasma membrane of various cells, primary and secondary active transporters may also be localized to the membrane of internal organelles. As an example of a primary active transporter, the V-type H+/ATPase (a proton pump) is localized to the membrane of endosomes, lysosomes, secretory vesicles, and synaptic vesicles, where it functions to hydrolyze ATP, and harnesses the energy released from ATP hydrolysis to transport proton (H+) into the lumen of the vesicle against its electrochemical gradient (proton electromotive force). Thus, the activity of the V-type H+/ATPase leads to an acidic lumen of vesicles (pH ≈ 5 versus pH ≈ 7.2 in the cytoplasm) and, in addition, leads to an inside positive membrane potential across the vesicle membrane (inside positive compared to the cytoplasm). As examples of secondary active transporters, H+/neurotransmitter exchangers use the H+ electrochemical gradient (proton electromotive force) established by the V-type H+/ATPase to drive neurotransmitter molecules against a concentration gradient into the lumen of secretory and synaptic vesicles. Examples include the vesicular monoamine transporter (VMAT), which transports monoamines such as serotonin, norepinephrine, epinephrine, and dopamine into synaptic vesicles. Another example is vesicular inhibitory amino acid transporter (VIAAT), which transports γ-aminobutyric acid (GABA) and glycine into synaptic vesicles. Additional examples include vesicular glutamate transporter (VGLUT), which transports glutamate into synaptic vesicles, and vesicular acetylcholine transporter (VAChT), which transports acetylcholine (ACh) into synaptic vesicles. It is important to note that each of the H+/neurotransmitter exchangers noted is expressed only in neurons differentiated to release that particular neurotransmitter. NT, neurotransmitter; H+, proton; ATP, adenosine triphosphate; ADP, adenosine diphosphate; Pi, inorganic phosphate.